Since the introduction of cellular communications services, the industry has experienced a rapid growth both in the area serviced and in the number of subscribers. Currently, in metropolitan areas of North America, networks of mobile switching centres (MSCs) each servicing over 100,000 call attempts per busy hour are becoming common. Each of these mobile switching centres services a network typically of over 100 cells and normally keeps track of the mobiles currently operating in its service area (SA) via a data base commonly referred to as a visitor location register (VLR). The visitor location register stores information regarding the services available to each subscriber, his home mobile switching centre and the current status (e.g. active or inactive) of the mobile. However the mobile switching centre does not as a matter of routine have accurate information regarding the precise geographical location of the mobiles. Consequently, when it is required to set up a call to a mobile, a page must be broadcast over the forward control channels (FOCCs), also known as the paging and access channels (PACH), to all the cells in the service area of the mobile switching centre. When the mobile responds to the page message the particular cell containing the mobile is then identified from the reverse control channel used for the mobile response and the call can be set up. If no response to the page is received it is assumed that the mobile is currently inactive and the call is treated accordingly.
Since, as discussed above, each mobile switching centre may serve a system of over 100 cells, in order to broadcast a page message over the forward control channels of all the cells in that system, the message is replicated into a corresponding number of copies by the mobile switching centre. As any response to the page will be received from only one cell there is a high degree of inbuilt redundancy in this method of system wide paging. The necessary message replication in the mobile switching centre is real time intensive thus reducing its call processing capacity.
One approach to this problem is to partition the cellular system into location areas which are considerably smaller than the typical service area of a mobile switching centre. Every time a mobile leaves one location area and enters another it registers a location update. The location area from which the mobile registered is recorded by the visitor location register so that, when the mobile is to be paged, the paging message is broadcast only to the cells of that location area. It will be appreciated that a reduction of the number of cells to be paged results in a corresponding reduction in the replication of the paging message. However there is also a consequent increase in registration activity in the mobile switching centre so that, although the system can be engineered to optimise the location area size so as to minimise the workload of the mobile switching centre, the improvement achievable by this technique is limited.
In one development of the above technique, the service area is partitioned into registration zones by broadcasting offsetting registration signals (REGIDs) in the different registration zones. For analogue mobiles the frequency of autonomous registration within a cellular system can be controlled via the overhead message train (OMT). Thus, if two adjacent cells belonging to different registration zones are broadcasting REGIDs that differ by a value equivalent to REGINCR this will prompt registration by the mobile whenever it moves from one cell to the other. The technique of setting up registration zones introduces a new problem associated with the re-scan process. Analogue mobiles re-scan the dedicated control channel set and lock on to the strongest channel immediately preceding the transmission of a registration message and also immediately following receipt of a registration acknowledgement message. In the circumstance where a mobile moves from a first cell in a first registration zone to a second cell in a second registration zone, it re-scans the channel set on crossing the cell boundary and locks on to the control channel of the second cell. The mobile immediately receives the overhead message train along with the registration information with the offsetting REGID prompting the mobile to register in the second cell. However, before the mobile transmits its registration message, it re-scans the control channel set and, because it is on the cell boundary, could (incorrectly) lock on to the control channel of the first cell thus transmitting the registration message over the reverse control channel of that cell. After receipt of the acknowledgement over the forward control channel of the first cell, the mobile may again lock on to the second cell, but will not register again because its memory registers have already been updated with the registration information received over the overhead message train of the second cell. Thus, although the mobile registered on crossing the cell boundary, the registration took place in the first zone instead of the second, and the information stored by the mobile switching centre regarding the location of the mobile is incorrect. This results in the paging message being broadcast in the wrong registration zone or location area with consequent failure of the attempt to set up a call.